A variety of techniques are used to perform intervention operations in subsea wells in the oil and gas industry. One technique is an “open water” technique in which a cable-type conveyance is run from a surface vessel into a subsea installation through the open water, e.g. water column, without using a riser. Typically, this type of intervention operation is limited to relatively shallow water.
In deeper water, which is often greater than 500 meters deep and often up to or above 3000 meters deep, numerous complications and challenges arise. For example, the length of conveyance moving through the water column can no longer be considered constant between the surface vessel and the subsea installation. Often, the conveyance assumes a bow-like shape due to a variety of effects including sea currents, surface vessel position and surface vessel movement. Additionally, the shape of the conveyance extending through the open water changes over the duration of the intervention operation.
Conventional cable-type conveyance operating techniques assume a known relationship between the length of the conveyance spooled out at the surface and the depth of an intervention well tool in the well. In deep, open water intervention operations, however, the relationship between conveyance movements at the surface and well tool movements within the subsea well are not necessarily known. The disconnect between surface movement of the conveyance and movement of the well tool through the well is due to the dynamic behavior of the conveyance extending through the water column and due to changes in environmental conditions.